1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus. More specifically, the present invention relates to a sheet conveying apparatus and an image forming apparatus, which convey a sheet absorbed onto an endless belt.
2. Description of Related Art
There has been an image forming apparatus, such as a copying machine, a facsimile, and a laser beam printer, which forms an image on a sheet by an electrophotographic system. When forming an image on a sheet, such image forming apparatus exposes a photosensitive drum according to image information to form an electrostatic latent image on the photosensitive drum.
The electrostatic latent image is developed with toner by a development device so as to be visible as a toner image. The toner image is transferred onto the sheet fed from a sheet feeding portion by a transfer portion. The sheet is then conveyed to a fixing nip between a fixing roller and a pressure roller provided in a fixing portion. The toner image is fixed as a permanent image onto the sheet.
The image forming apparatus has a sheet conveying apparatus which conveys a sheet. The sheet conveying apparatus has a first sheet conveying portion, and a second sheet conveying portion which conveys the sheet absorbed by air to the first sheet conveying portion. The second sheet conveying portion has an endless belt, moves the endless belt on which the sheet is placed, and conveys the sheet to the first sheet conveying portion.
The sheet simply placed on the conveying belt is fluttered while being conveyed. Consequently, the conveyance can be unstable to cause conveying failure. In addition, an unfixed toner image formed on the surface of the sheet can be irregular to cause image failure.
To prevent such sheet fluttering to perform stable conveyance, in Japanese Patent Application Laid-Open Nos. H01-104560 and H09-43913, there is provided the second sheet conveying portion which conveys a sheet absorbed onto the conveying belt.
FIG. 10 is a diagram illustrating the configuration of the related art sheet conveying apparatus having such second sheet conveying portion which conveys a sheet absorbed onto the conveying belt. A second sheet conveying portion 204 has an endless belt 200 holding a sheet S, and an air suction member 201 which generates a negative pressure inwardly of the entrained surface of the endless belt 200 as a member which prevents the fluttering of the sheet S. The air suction member 201 has a duct portion 202, and a suction fan 203 coupled to the duct portion 202.
When the sheet S is conveyed, the suction fan 203 is rotated to generate a negative pressure in the duct portion, so that the sheet S is absorbed onto the endless belt 200 and is then conveyed to a fixing portion 300 configuring the first sheet conveying portion by preventing fluttering of the sheet S. According to the type of the sheet S, the suction force of the suction fan 203 is changed to secure the stable absorption force without depending on the size of the sheet S. Stable sheet conveyance can be performed by preventing fluttering without making a toner image irregular.
In such sheet conveying apparatus and the image forming apparatus having the same, in the case of a sheet larger than A4-size, a sheet trailing end is placed on the endless belt 200 until the sheet reaches the fixing portion 300. As a result, the sheet S can be conveyed to a fixing nip N between a fixing roller 302 and a pressure roller 303 of the fixing portion 300 without decreasing the absorption force.
On the other hand, in the case of a small-size sheet, such as a B5-size sheet and a postcard, a sheet trailing end can reach the endless belt 200 before a sheet leading end reaches the fixing portion 300. Thereafter, a sheet S′ is conveyed. As illustrated in FIG. 10, the sheet trailing end passes on the endless belt, and the sheet S′ is placed on part of the endless belt 200, not on the whole endless belt.
Air indicated by arrows taken in from a portion L2 of the endless belt 200 on which the sheet S′ is not placed becomes significant. Accordingly, the absorption force in a leading end portion L1 of the endless belt 200 on which the trailing end of the sheet S′ is placed is decreased to reduce the conveying force. When the conveying force is reduced, the sheet S cannot be pushed into the fixing nip N, and the sheet S′ is jammed.
In FIG. 10, a fixing inlet guide 301 is provided between the fixing portion 300 and the second sheet conveying portion 204. The sheet is bent at an angle α just before the fixing nip N by the fixing inlet guide 301 and is then moved toward the fixing nip N. In this state, when the sheet passes through the fixing nip N, the wrinkling and jamming of the sheet can be prevented.
However, the conveying load is increased by providing the fixing inlet guide 301. Consequently, the sheet S is easily separated from the endless belt. The leading end of the sheet S guided by the fixing inlet guide 301 abuts the fixing roller 302. Due to the abutting shock, the sheet S is easily separated from the endless belt 200.
As indicated by a dotted line, if the small-size strong sheet S′ is hard to be guided by the fixing inlet guide 301 at the bending angle α, the sheet S′ is easily separated from the endless belt 200. The abutting shock is easily to be transmitted to the trailing end of the small-size sheet S′ to make the sheet conveying force unstable. Further, when the fixing inlet guide 301 becomes dirty by unfixed toner or sheet particles in terms of durability to increase the friction resistance, the conveying force is further insufficient, and the sheet conveying force becomes unstable.
To solve this problem, the suction force of the suction fan 203 as a negative pressure member is increased corresponding to the small-size strong sheet. Then, the operating noise of the suction fan 203 is increased, and the power consumption is wastefully used. When the number of fans is increased along the sheet conveying direction corresponding to the sheet size in the conveying direction, the apparatus becomes larger. Similarly, due to poor suction efficiency, the power consumption is increased.
The present invention has been made in view of such conditions and provides a sheet conveying apparatus and an image forming apparatus, which can stably convey a sheet without depending on sheet size.